A muffler is a device used to attenuate sound propagated in conjunction with a moving stream of fluid, usually a gas. Mufflers generally fall into two categories depending on how the sound energy is removed from the gas stream: reactive and dissipative. A reactive muffler, also known as a nondissipative muffler, attenuates the sound energy by reflecting the sound back toward the source. A dissipative muffler absorbs the sound energy as the gas passes through the muffler. Design considerations for the acoustical performance of a motor vehicle muffler include: (1) the required sound attenuation as a function of frequency and length, (2) the effect on the exhaust gas flow and resulting system backpressures, and (3) the economics of manufacturing and installation.
The disadvantages of dissipative mufflers are numerous. Unburned carbon particles tend to close the pores of sound absorbing materials lining the walls of the muffler. The high velocity unsteady flow of exhaust gasses blows out the fibers of the absorptive lining. Thermal cracking of the linings frequently occurs. There is poor attenuation at low frequencies, i.e., on the order of the firing frequency, where most of the exhaust noise is concentrated. Finally, there are relatively higher manufacturing costs as compared with a reactive muffler.
Most of the noise from a motor vehicle engine is at the firing frequency and the first few harmonics. Exhaust noise from motor vehicles generally consists of (1) sound generated when combustion gasses leave the engine manifold and (2) sound generated when the exhaust gasses flows through the exhaust pipe. The first sound is in the form of pulsating pressure waves that include frequency components proportional to the engine speed. The first sound therefore has a relatively large amount of low frequency components. The second sound has a relatively large amount of high frequency components. Low frequency noise components are easily muffled with a modest size muffler. Motorcycles present a challenge to the noise engineer due to the limited muffler space available.
Motor vehicle mufflers are predominantly of the reactive type. Reflection is provided through acoustic filters, resonators, and changes in direction caused by bends in the pipe containing the gas stream. Reactive mufflers are useful in low frequency applications where the high temperatures or flammable exhaust gasses restrict the use of dissipative materials. A primary characteristic of reactive mufflers is a relatively high pressure drop for a given value of gas flow velocity. This pressure drop exhibits itself as a back pressure at the exhaust of the engine, thereby restricting the engine performance. Back pressure is the extra static pressure exerted by the muffler on the engine through restriction in the flow of exhaust gasses.
Conventionally, the muffler volume is proportional to the engine piston displacement and inversely proportional to the engine speed and square root of the engine cylinders. This can be represented as: ##EQU1##
where K values range from 5,000 for farm tractors, 1,000 for off-highway trucks and heavy equipment, 35,000 for highway trucks, up to 50,000 for passenger cars.
The fundamental frequency of piston-engine exhaust noise in the exhaust line is the product of the number of cylinders firing per revolution and the engine speed, assuming the exhaust manifold has a center outlet. If the exhaust manifold has an end outlet instead of a center outlet, the frequency is reduced by half. For example, a 6 cylinder 4-stroke cycle engine operating at 3,000 rpm has a fundamental exhaust frequency of (6/2)(3,000/60)=150 Hz. The critical length of the exhaust pipe depends on the fundamental exhaust frequency and the mean temperature of the exhaust gasses. The critical length is .lambda./2 and all integer multiples of .lambda./2 (harmonics), where .lambda. is the wavelength of the sound in the exhaust pipe An exhaust muffler of the critical length sets up a standing wave with maximum pressure at the exhaust valves and minimum pressure at the end of the exhaust pipe. Assuming that the exhaust gas temperature in the exhaust pipe is such that the velocity of sound is 1,500 fps, then the critical length of this engine is 1/2 of 1,500/150=5 feet. Thus, 5 feet, 10 feet, etc. are critical lengths for exhaust lines in this engine.
The usual length to diameter ratio, l/d, is about 4:1, but can be as high as 8:1 in straight-through mufflers. A small l/d ratio muffler attenuates the sound well for a narrow frequency band, whereas a large l/d ratio muffler attenuates the sound over a wider frequency band but not as well.
Exhaust noise is appreciably reduced by filtering (friction effects) and using resonance chambers to offset the noise-wave effects. The total aeroacoustic attenuation in a moving medium (exhaust gasses) is a sum of the viscothermal effects and turbulent flow friction. A simple expansion chamber 1 in a muffler 9 as shown in FIG. 1A is effective for one relatively low noise frequency. Some friction is also present due to a relatively small exit hole 2 in an exit plate 3. FIG. 1B shows a baffle muffler 9 with control holes 4 in each baffle 5 that introduce friction effects. A plurality of chambers 6 are resonance chambers which have a very high frequency and are effective for filtering a narrow band of high sound frequencies. FIG. 1C shows a straight-through muffler which has one resonating chamber 7 connected to a central perforated pipe with a plurality of perforation holes 8 but no baffles or associated friction effects. FIGS. 1D and 1E show combinations of baffles 5 and resonator chambers 7. FIG. 1F shows four resonator chambers 7 of different frequencies which depend on the ratio of perforation area from perforation holes 8 to resonator volume. The higher this ratio, the higher the frequencies that are attenuated.
In general, torque is the ability of an engine to gain rpms, while horsepower is how much power the engine produces at a given rpm. Increasing backpressure increases torque in the low to mid-range rpms. After that, the torque decreases with increased backpressure. However, at mid-range rpms and higher, the horsepower decreases as the backpressure increases. The mid-range rpms thus affect torque and horsepower in different ways. In resistive muffler design, the general tradeoff is that, as surfaces that reflect noise back toward the engine are increased (in order to reduce the noise), the overall back pressure experienced by the system increases. Decreasing the back pressure usually increases the noise. Increased engine backpressure affects the engine timing and power output, as well as increasing unwanted exhaust pollutants.